Optical scanning devices that scan light are widely used in digital copiers, laser printers, bar code readers, scanners, projectors, and the like. As this optical scanning device, conventionally a polygon mirror or galvanometer mirror that uses a motor has generally been used.
On the other hand, with the developments in ultra-fine processing technology in recent years, optical scanning devices manufactured by applying MEMS technology have made significant advances. Among these, an optical scanning device that scans light by causing an oscillating mirror to oscillate in a reciprocating manner with a beam unit serving as a rotating shaft has been attracting attention. Compared with a conventional optical scanning device that uses rotation of a polygon mirror or the like using a motor, due to an oscillating mirror that is formed by MEMS technology having a simple structure and integral molding by a semiconductor process being possible, there are the advantages of miniaturization and cost reduction being easy, and speeding up being easy due to the miniaturization.
In an oscillating mirror that utilizes MEMS technology, the drive frequency and the resonance frequency of the structure are generally made to match in order to increase the oscillation angle. The resonance frequency fr of the mirror is given by the following equation from the torsion spring constant k of the beam unit, and the inertia moment IM of the oscillating mirror.fr=1/(2π√(k/IM))  (1)
With the width of the beam unit being w, the thickness t, the length L, and assuming t<w, the torsion spring constant k in Equation (1) is given by the following equation.k=(Gβtw3)/L  (2)
Here, G is the transverse elasticity constant, and is represented by G=E/(2(1+v)), using the Young's modulus E and the Poisson's ratio v of the material that forms the beam unit. β is a constant determined from the ratio of w and t of the beam.
At the time of oscillation of the oscillating mirror, the beam unit undergoes torsional deformation at high speed and for a long time. However, since the beam unit and the oscillating mirror are integrally molded with single-crystal silicon, it is considered to possess sufficient endurance to this deformation.
Thus, the resonance frequency is determined from the inertia moment of the oscillating mirror and the torsion spring constant of the beam unit and the like. However, on the other hand, it is not possible to avoid variations in these values due to differences in the processing accuracy and ambient temperature. For that reason, variations also occur in the resonance frequency.
Therefore, in order to solve the problem mentioned above, an optical scanning device has been proposed in which an adjusting mechanism for the resonance frequency of the oscillating mirror is provided. With the adjusting mechanism of this optical scanning device, it is possible to adjust fluctuations of the resonance frequency due to variations in the processing accuracy of members and changes in the ambient temperature, and to keep the resonance frequency constant.
As such a constitution, for example Patent Document 1 discloses a resonance-type optical scanner that has a first beam unit, a second beam unit, a first piezoelectric element unit, and a power supply unit. The first beam unit is coupled to one end of the oscillating mirror. The second beam unit is coupled to the other end of the oscillating mirror. The first piezoelectric element unit causes the first beam unit to undergo elastic deformation. The power supply unit applies a voltage for driving the oscillating mirror to the first piezoelectric element unit. This resonance-type optical scanner, by the first supply unit applying a direct voltage component to the first piezoelectric element unit to produce a tensile force in the first beam unit and the second beam unit, changes the modulus of elasticity of the beam units, and performs adjustment of the resonance frequency.
However, in Patent Document 1, the piezoelectric element (metal thin film or ceramic polycrystalline body) that is laminated on the surface of the beam unit is directly influenced by the torsional deformation of the beam units during resonance, and so defects occur from the grain boundary, and a fatigue breakdown easily occurs. That is to say, the problems occur of the adjustment accuracy of the resonance frequency falling, and adjustment no longer being possible.
In contrast to this, Patent Document 2 discloses a device that includes a first beam unit that is coupled to one end of an oscillating mirror, a second beam unit that is coupled to the other end of the oscillating mirror, and a first structure for causing the first beam unit to undergo elastic deformation. This device produces tensile force in the first beam unit by applying a voltage to the first structure, to perform adjustment of the resonance frequency. In this case, the oscillating mirror is assumed to be driven by electrostatic force with electrodes arranged on the lower unit or side surfaces of the mirror.